1. Field of Invention
The invention relates to a sewing machine having multiple needle bars and needle thread take-ups which are used in a sequential manner enabling creation of a sewing pattern in which multiple colors are used.
2. Description of Related Art
Conventionally, there are sewing machines having a plurality of needle bars and needle thread take-ups on a machine head, which are changed one by one successively to continue a sewing operation. These examples are indicated in Japanese laid-open Patent Publication Nos. 9-38367 and 9-75569.
When one such sewing machine is used for creating a colorful sewing pattern stitched with various color threads, it can continue a sewing operation while automatically changing needles without interruption so that operator does not have to change threads.
However, speeding up of the sewing machine has been recently tried to produce sewing operations with high efficiency. For an embroidery machine, for example, the speed of the upper shaft is limited to about 1,000 to 1,200 rpm, but it is expected to be higher.
Where multiple needle bars are installed in a sewing machine, all needle positions are not aligned because of slight differences, such as machining accuracy and assembly accuracy. As a result, a relative position between a needle attached to the bottom of a needle bar and a rotary hook often varies for each needle. Therefore stitching quality deteriorates when the machine is in operation at a higher speed.